1. Field of the Invention
The present invention relates to monitoring tags, and in particular to tags that can be secured with a strap to an item so that a location or security status of the item can be monitored. For example, the monitoring tag might be secured around the appendage of an individual or animal, or to part of an object or item of merchandise.
2. Related Art
The monitoring tags of the present invention can include a location detection means for detecting if the item has moved outside a designated area within which a wireless signal can be exchanged between the monitoring tag and a base unit. As such, the tags can be worn by hospital patients, the elderly, children or offenders and can raise an alarm or notify an authorised person if they move outside the designated area. The tags can also be used in other situations such as stock control, employee location control or to detect if a valuable object has been moved, for example. The monitoring tags may include means to try and prevent tampering and where this is not possible, to provide an indication that tampering has taken place.
These tamper detection means can be provided in addition to, or instead of, the location detection means. The tamper detection means can be used to raise an alarm or notify an authorised person if the strap that is used to secure the monitoring tag is cut, detached from a housing of the tag or interfered with in any way.
Monitoring tags are known where a housing is secured around the appendage of an individual by means of a flexible elongate strap. The housing includes a radio frequency transmitter that can exchange signals with a base unit. Such communication between the monitoring tag and the base unit can only take place when the monitoring tag is within range. Consequently, if the monitoring tag is taken too far away from the base unit then the communication will cease. The lack of communication can be automatically recorded or logged by the base unit or the base unit can trigger an alarm or notify an interested party.
The housing can also include a tamper detection circuit. A typical tamper detection circuit might include an optical transmitter for sending optical signals along an optical fibre (sometimes called a fibre-optic cable) embedded in the strap. If the strap is intact and properly secured to the housing then the optical signals will travel along the optical fibre and can be detected at the other end by an optical receiver. However, tampering with the strap by cutting it or detaching it from the housing will interrupt the optical signal and trigger the optical tamper detection circuit.
One known way of trying to remove the monitoring tag is to heat the strap and stretch it. In many cases the optical fibre will stretch with the rest of the strap so there is no critical interruption of the optical signal. To prevent this from happening, a thread, yarn, braid or tape of a thermally inelastic material such as Kevlar® material (supplied by DuPont Corporation of Wilmington, Del., United States of America) can be embedded in the strap. However, the applicant has found that unless this thread, yarn, braid or tape is mechanically bonded to the rest of the strap and/or mechanically secured to the housing (either directly or indirectly through the clips mentioned below) then it is still possible to stretch the strap because there is relative movement between the thermally inelastic material and the surrounding material of the strap.
The strap is usually made a snug but comfortable fit around the appendage of the individual. The strap must also be capable of being detached from the housing in case of an emergency or when the monitoring tag is legitimately removed. The strap is therefore often provided in a single piece and secured to the housing a pair of clips. The applicant has found that it is not always obvious if the clips have been tampered with in an attempt to remove the monitoring tag.
The applicant has also found that known monitoring tags may suffer from problems caused by the ingress of liquids through openings in the housing for receiving the ends of the strap or the clips. If the liquid gets between the ends of the optical fibre and the optical transmitter or optical receiver of an optical tamper detection circuit then it can interfere with the proper detection of the optical signal, thereby causing the optical tamper detection circuit to trigger unnecessarily. The liquid can also damage the optical fibres and the other electronic components inside the housing.